1. Field of the Invention
This invention relates to a process for preparing terephthalic acid of high purity, more particularly it relates to a process for preparing terephthalic acid of such high purity that permits it to be reacted directly with a glycol for the production of a polyester.
2. Description of the Prior Art
Terephthalic acid is useful as a starting material for polyesters and has usually been prepared by the so-called SD method wherein p-xylene is oxidized with molecular oxygen in an acetic acid solvent in the presence of a heavy metal-containing catalyst. However, the terephthalic acid product obtained by the SD method usually contains a relatively large amount (on the order of 1,000 to 3,000 ppm) of by-product 4-carboxybenzaldehyde (hereinafter referred to as 4CBA") and therefore it cannot be used as a starting material for the production of polyesters for use as fibers, films and the like as it is.
As a result, in the prior art technique the terephthalic acid is reacted with methanol to convert it into dimethyl terephthalate, which is then purified and reacted with a glycol. Alternatively, in recent years, there have developed processes for the purification of the crude terephthalic acid produced by the SD method which comprise dissolving it at an elevated temperature and pressure and contacting the resulting solution with a noble metal catalyst such as palladium, thereby providing a highly pure terephthalic acid containing not more than 30 ppm of 4CBA. However, the former purification technique is disadvantageous in that methanol is inevitably formed during the process of polyester production because of the use of dimethyl terephthalate as one of the starting materials for polyesters. While preferred in that the terephthalic acid can be purified as such, the latter technique requires two separate plant systems because there are differences in the solvents, catalysts and operating conditions employed between the preparation and purification processes of terephthalic acid.
More recently, an attempt has been made to prepare a highly pure terephthalic acid directly in a single plant by employing a particular combination of catalyst, oxidation conditions and oxidation mode. For example, a so-called post-oxidation technique has been known as a means for improving the purity of the terephthalic acid products prepared in oxidation plants of p-xylene. According to this technique, the slurry of terephthalic acid discharged from the oxidation reactor in which p-xylene is oxidized is treated with molecular oxygen at a relatively high temperature before the desired terephthalic acid is recovered from the slurry by solid-liquid separation. The purpose of this treatment is the removal of any intermediate, coloring contaminant and other impurity. More specifically, there is disclosed treatment of the slurry of terephthalic acid with molecular oxygen at a temperature above the reaction temperature (Japanese Patent Publication No. 12695/65) or at a temperature below the reaction temperature (Japanese Patent Laid-open (KOKAI) Nos. 16630/76, 39642/76,85136/77 and 37636/78). Another unique technique in which the post-oxidation is carried out intermittently in a batchwise operation has also been disclosed (Japanese Patent Laid-open No. 31947/72).
Among these techniques, the post-oxidation effected at a temperature below the reaction temperature produces the most favorable results and affords the desired product of relatively high purity as compared with the terephthalic acid products obtained by the conventional process comprising only a single-step oxidation of p-xylene. However, the terephthalic acid products obtained by such lower temperature post-oxidation technique still contain 100 to 500 ppm of 4CBA, which is the limit of 4CBA level attained in commercial-scale operation of this technique. Although such terephthalic acid products can directly be used in polymerization with a glycol without further purification, but they are of medium purity and are inferior in purity to those products which have been subjected to the above-mentioned purification treatment with palladium or other catalyst.
Therefore, there is a continuing need for more economically attractive processes for the preparation of terephthalic acid of high purity in a single plant.